1. Field of the Invention
The present invention relates to hydrogenation of silicon substrates in general and in particular to an improved hydrogenation method for passivating defects in silicon solar cells.
2. Background of the Invention
A typical silicon solar cell comprises a p-type and an n-type conductivity region in a silicon substrate and generates electricity when impinging radiation creates electron-hole pairs that migrate to the respective p-type and n-type conductivity regions. Unfortunately, certain impurities in the silicon substrate, as well as defects in the crystal lattice itself, can reduce the efficiency of the solar cell over what otherwise could be achieved if the silicon substrate were perfect and defect free. Consequently, researchers have searched for ways to minimize the effects of such defects and impurities, thus increasing the overall efficiency of the cell.
One method to minimize the effects of the defects and impurities is to hydrogenate the cell by implanting hydrogen into the silicon substrate. Although the exact effect that hydrogen has on the substrate is not yet fully understood, it is known that the hydrogen minimizes or "passivates" the defects and impurities in the substrate. For example, many of the unit cells in the crystal lattice may be missing one or more silicon atoms, creating voids or areas where silicon atoms should be, but are not. Hydrogenating the substrate allows hydrogen atoms to fill those voids in the unit cells, thereby reducing the deleterious effects of those voids or defects. In fact, it is now well established that the performance of solar cells, as well as many other electronic devices that are fabricated on p-doped and n-doped silicon substrates, such as integrated circuits, can be improved by such hydrogen passivation. With regard to solar cells, hydrogen passivation improves cell efficiency by reducing minority carrier recombination losses at grain boundaries, dislocations, and other defects in the crystal lattice. Unfortunately, while the improved performance resulting from hydrogen passivation has been demonstrated in the laboratory, it has been difficult to apply hydrogen passivation techniques commercially, because the precise effects of hydrogenation on the silicon substrate still are not well understood.
In the past, solar cells have been hydrogenated by introducing hydrogen into silicon substrate from the junction or front side of the cell by using an RF plasma or by ion implantation. However, front-side hydrogenation is not without its disadvantages. For example, all the cell processes that require temperatures in excess of 300.degree. C. must be carried out prior to hydrogenation, since temperatures above about 300.degree. C. will drive the previously implanted hydrogen out of the substrate. Moreover, because hydrogen ions do not readily pass through the anti-reflective (AR) coatings usually placed on the front-side of the cell, front side hydrogenation also must be performed prior to the deposition of the AR coating. Therefore, the subsequent AR coating process must be carried out at a temperature below 300.degree. C., which, unfortunately, requires the use of more expensive coatings and processes. Also, such coatings and processes make it more difficult to achieve good AR coatings over large areas. Another significant disadvantage associated with the front-side hydrogenation process is that it creates a damaged surface layer on the front side of the substrate. The actual thickness of the damaged surface layer is relatively small compared to the total thickness of the substrate, but the damaged surface layer nevertheless reduces cell performance.
Recognizing the disadvantages associated with front surface hydrogenation, researchers have developed various processes to hydrogenate the substrate from the back-side. A primary advantage of back side hydrogenation is that the damaged surface layer is created on, and confined to, the back side of the substrate, away from the junction side of the cell, thus minimizing its adverse effect on cell performance. Another advantage associated with back-side hydrogenation is that it can be performed after the deposition of the front-side AR coating, since the hydrogen ions are implanted from the back-side of the cell. Unfortunately, however, back side hydrogenation is not without its drawbacks. For example, back-side hydrogenation requires either a fully open or a partially open back contact, so the hydrogen can be implanted into the substrate. Another disadvantage is that it is still necessary to ensure that the subsequent processing steps, such as annealing or alloying, are carried out at temperatures below 300.degree. C. to prevent the hydrogen from escaping from the substrate.
One way around the temperature limitation described above is to coat the back side of the cell with a hydrogen-encapsulating material to prevent the escape of the implanted hydrogen during subsequent high-temperature processing. In my paper entitled A Backside Hydrogenation Technique for Defect Passivation in Silicon Solar Cells, J. Appl. Phys. Vol. 64, 15 November 1988, pp. 5264-5266, I described a process to confine the hydrogen in the silicon substrate during the subsequent back-side contact alloying process. Essentially, that process required the deposition of a layer of aluminum about 2000 .ANG. thick on the back side of the cell to prevent the hydrogen from escaping during subsequent high-temperature processing. Although the foregoing process attempted to maximize the then-recognized benefits of substrate hydrogenation, the aluminum coating process represented an additional step that did not directly add to cell efficiency. Also, at that time it was not generally recognized that the hydrogenation process, while effectively reducing the minority carrier recombination losses, introduced its own performance degrading effects on the solar cell, which effects should be minimized to maximize cell efficiency.
Therefore, there is a need for an improved hydrogen passivation process that maximizes the passivation effect on defects and impurities in the silicon substrate, while at the same time minimizing the deleterious effects associated with hydrogenation itself. Ideally, such a process should be able to be performed on completed solar cells and with a minimum number of steps to minimize cell production costs. Until this invention, no such process existed.